US heading for point when 'military pursuit of al-Qaida should end'

Fight against terrorist group on course for Obama to stop using legal authority given by Congress to wage war, says lawyer

US defence department general counsel, Jeh Johnson, says responsibility for tackling al-Qaida should pass to the police and other law enforcement agencies when the 'tipping point' in pursuit of group is reached. Photograph: Alex Wong/Getty Images

The US is heading for a "tipping point" beyond which it should no longer pursue al-Qaida terrorists by military means, one of the Obama administration's most senior lawyers has said.

Jeh Johnson suggested the group would become so degraded that a time would come when the legal authority given to the White House by Congress should no longer be used to justify waging the war that has been fought since 2001.

Johnson said that when this happened, America had to "be able to say ... that our efforts should no longer be considered an armed conflict against al-Qaida and its affiliates".

Instead, the responsibility for tackling al-Qaida should pass to the police and other law enforcement agencies.

Johnson has been general counsel at the US defence department for the past four years and has given advice on every military operation that needs the approval of the president or defence secretary.

In a speech presented tonight in the UK, Johnson was expected to set out the legal principles underpinning the conflict against al-Qaida and insisted they were rooted in domestic and international law. Congress had authorised the president to use "all necessary and appropriate force" against the nations, organisations and individuals responsible for the 9/11 attacks; the US supreme court had endorsed this in 2006 by ruling "our efforts against al-Qaida may be properly viewed as armed conflict".

But Johnson also made clear these principles were not open-ended, and that the US government would need to respond when circumstances change. And though he said he could not predict when the conflict would draw to a close, he said the US must not be afraid to change its tactics.

"I do believe that on the present course there will come a tipping point, a tipping point at which so many of the leaders and operatives of al-Qaida and its affiliates have been killed or captured, and the group is no longer able to attempt or launch a strategic attack against the United States, such that al-Qaida as we know it, the organisation that our Congress authorised the military to pursue in 2001, has been effectively destroyed.

"At that point we must be able to say to ourselves that our efforts should no longer be considered an armed conflict against al-Qaida and its associated forces, rather a counter-terrorism effort against individuals who are the scattered remains of al-Qaida … for which the law enforcement and intelligence resources of our government are principally responsible." America's military assets would then be available in reserve, he said.

The US could not "capture or kill every last terrorist who claims an affiliation with al-Qaida" and the enemy "did not include anyone solely in the category of activist, journalist, or propagandist".

Such characterisations will put Washington under greater pressure to review and justify the military campaign against al-Qaida, which has been virtually wiped out in Afghanistan and Pakistan, and now exists only in small, disorganised regional splinter groups.

Speaking at the Oxford Union, Johnson insisted the US was applying conventional law to an unconventional enemy, and justified detaining prisoners indefinitely and using "targeted lethal force" – such as drones – to kill suspects. He conceded these techniques would be questionable "viewed in the context of law enforcement or criminal justice, where no person is sentenced to death or prison without an indictment, an arraignment, and a trial before an impartial judge and jury".

But, he added: "Viewed within the context of conventional armed conflict, as they should be, capture, detention and lethal force are traditional practices as old as armies. Capture and detention by the military are part and parcel of armed conflict. We employ weapons of war against al-Qaida but in a manner consistent with the rule of law. We employ lethal force, but in a manner consistent with the law of war principles of proportionality, necessity and distinction."

Johnson said that when the military conflict came to an end, those still in detention would not necessarily be released immediately. He said that after the end of the second world war, the US and British governments delayed the release of some Nazi prisoners of war.

"We refuse to allow this enemy, with its contemptible tactics, to define the way in which we wage war," he said. "Our efforts remain grounded in the rule of law."